A hydraulic circuit shown in FIG. 6 is employed in a conventional construction machine such as a hydraulic excavator and the like. The hydraulic circuit is provided with a variable capacity pump 11 (hereinafter, referred to as a pump 11) driven by an engine 1 and a pilot pump 81. An angle of a swash plate in the pump 11 is controlled by a servo piston 12, and an operation pressure of the servo piston 12 is controlled by a servo control valve 200. An operation portion 200a of the servo control valve 200 is connected to a neutral control valve 210 (hereinafter, refer to as an NC valve 210), a cut-off valve 220 (hereinafter, refer to as a CO valve 220) and a torque variable control valve 230 (hereinafter, refer to as a TVC valve 230) in series. A pipe passage 202 branched from a discharge pipe passage 201 of the pump 11 is connected to each of the operation portions in the CO valve 220 and the TVC valve 230. A pipe passage 222 branched from a discharge pipe passage 221 of the pilot pump 81 is connected to the operation portion 200a in the servo valve 200 via the TVC valve 230, the CO valve 220 and the NC valve 210. An engine rotation sensor 5 for detecting a rotational speed of the engine 1 is connected to a controller 240. The controller 240 is connected to the TVC valve 230.
Further, the discharge pipe passage 200 of the pump 11 is connected to a direction switch valve 250. The direction switch valve 250 is connected to a hydraulic cylinder 260 via pipe passages 251a and 251b and also connected to a jet sensor (a pressure detecting portion) 253 via a pipe passage 252. The jet sensor 253 is connected to a drain passage 254.
Still further, a discharge pipe passage 223 branched from the discharge pipe passage 221 in the pilot pump 81 is connected to a pressure proportional control valve 270 and an operation lever 271 is connected to the pressure proportional control valve 270. The pressure proportional control valve 270 is connected to an operation portion of the direction switch valve 250 via pipe passages 272a and 272b.
An operation of the controller mentioned above will be described below. The NC valve 210 receives a pressure detected by the jet sensor 253 at an operation portion in one side from a pipe passage 256 and receives a pressure detected by the drain pipe passage 254 disposed downstream the jet sensor 253 at an operation portion in the other side from a pipe passage 257, thereby being switched in accordance with a differential pressure between a front and a back of the jet sensor 253. Since all the discharge flow amount of the pump 11 is drained from the drain passage 254 to a tank 258 via the jet sensor 253 when the direction switch valve 250 becomes a neutral position shown in the drawing, a pressure downstream of the jet sensor 253 becomes high and the NC valve 210 is switched to a port position 210b. Accordingly, the servo valve 200 becomes at a port position 200c so as to move the servo piston 12 to a left side in the drawing and reduce a flow amount of the pump 11. Therefore, an energy loss at a neutral position of the direction switch valve 250 is reduced.
Next, since no oil flows in the jet sensor 253 when an operator switches the direction switch valve 250, the NC valve is switched to the port position 210a. Further, a rotational speed signal from the engine rotation sensor 5 in the engine 1 is always input to the controller 240, and a command signal is input to the operation portion 230a of the TVC valve 230 from the controller 240 in correspondence to the rotational speed signal. In this case, a discharge pressure of the pump 11 is input to the operation portion 230b of the TVC valve 230.
In this case, when the discharge pressure of the pump 11 is lower with respect to the command signal of the engine rotational speed signal, the port position of the TVC valve 230 is switched to a position indicated by 230c and the CO valve 220 is switched to a position indicated by 220a. Since the NC valve 210 is at the port position 210a as mentioned above and the pilot pressure from the pipe passage 222 is accordingly input to the operation portion 200a of the servo valve 200, the servo valve 200 is switched to a position indicated by 200b. Therefore, oil in the side of a head of the servo piston 12 is drained, pressurized oil from the pipe passage 222 is flowed into a bottom side, and the servo piston 12 moves rightward so as to increase the pump discharge amount.
On the contrary, when the discharge pressure of the pump 11 is higher with respect to the command signal of the engine rotational speed, the TVC valve 230 is switched to a position indicated by 230d and the pilot pressure from the pipe passage 222 is not input to the operation portion 200a of the servo valve 200, so that the servo valve 200 is switched to the position indicated by 200c. Therefore, the pressurized oil from the pipe passage 221 is flowed into the head side of the servo piston 12, oil in the bottom side is drained, and the servo piston 12 moves leftward so as to reduce the pump discharge amount.
Since the CO valve 220 is structured such that a force of a spring 220b is set to be larger in comparison with the discharge pressure of the pump 11, it is normally at the position indicated by 220a. Further, the CO valve 220 is structured such as to be switched to the position indicated by 220c when the pump 11 reaches a maximum pressure and is also structured such as to perform a cut off control for further reducing a flow amount under the maximum pressure.
The TVC valve 230 is structured such as to control so that a discharge flow amount Q [Q=q(cc,/rev).multidot.N] of the pump 11 becomes constant in correspondence to an engine rotational speed N and a discharge pressure P of the pump 11, and an absorption horsepower of the pump 11 is controlled on a constant line having a substantially equivalent horsepower P.multidot.Q=Constant as shown by a dotted line Hs in a P-Q graph in FIG. 3.
Recently, in order to increase a work force and a work speed in correspondence to a load condition of a work, it is structured such as to change the P-Q graph in FIG. 3 and change a matching point (A1) between the engine output torque and the pump absorption torque as shown in FIG. 2.
For example, a controller which the applicant suggests in Japanese Patent Application No. 7-46508, comprises active mode selecting and canceling means having an engine, a variable capacity pump driven thereby, pump output control means for controlling so that the product of a load pressure acting on the pump by a discharge volume becomes substantially constant, a work apparatus receiving a pressurized oil from the pump and operated by an actuator and a switch selecting an engine output torque and a pump absorption torque in accordance with a work and performing a heavy excavation and the like, engine fuel injection position setting means for supplying a fuel by which the engine outputs a rated output torque in accordance with a selection of an active mode, active mode switching means for switching a set pressure of a relief valve for adjusting an oil pressure to the actuator in accordance with the selection of the active mode, a safety valve and the like, and control valve for outputting a command to the engine fuel injection position setting means and the active mode switching means by receiving a signal from the active mode selecting and canceling means.
However, in some condition of a work field or some work load condition, it is desired to increase up the work force and the work speed further in comparison with the active mode.
Accordingly, it is necessary to increase the engine output, the engine rotational speed, and the main relief set pressure of the hydraulic circuit in the working machine further in comparison with the active mode at a time of a heavy excavating operation. In accordance with this, the control of the matching point between the engine output torque and the pump absorption torque is changed, whereby the work force and the work speed is further increased, so that a controller in which a speedy work can be performed even under a heavy load and a strong work. at the critical moment can be performed is required.